“Slide-ring gels”, new gels different from physical gels and chemical gels, have been developed in recent years. A compound that is used for such slide-ring gels and is drawing attention is a crosslinked polyrotaxane.
Crosslinked polyrotaxanes are produced by crosslinking polyrotaxanes in which a capping group is introduced at each end of a pseudopolyrotaxane. In the case that a pseudopolyrotaxane is formed from a polyethylene glycol (hereinafter, also referred to as a “PEG”) having a reactive group at each end and a cyclodextrin that includes the PEG, for example, the resulting crosslinked polyrotaxane has a structure in which linear molecules of the PEG are threaded through cyclodextrin molecules in a skewered manner and the cyclodextrin molecules are movable along the linear molecules (has a pulley effect). The pulley effect allows the crosslinked polyrotaxane to uniformly distribute tensile force applied thereto. The crosslinked polyrotaxane is therefore not likely to have cracks or flaws, i.e., has excellent characteristics that conventional crosslinked polymers do not have.
Polyrotaxanes used for production of a crosslinked polyrotaxane typically contain isolated cyclodextrin molecules (hereinafter also referred to as “free cyclodextrin molecules”). These free cyclodextrin molecules deteriorate the characteristics of a crosslinked polyrotaxane. Hence, polyrotaxanes need to be purified by a method such as reprecipitation such that free cyclodextrin molecules are removed.
Patent Literature 1 describes a method for producing a polyrotaxane which includes mixing a carboxylated polyethylene glycol and a cyclodextrin molecules to obtain a pseudopolyrotaxane with a carboxylated polyethylene glycol included in the cavities of the cyclodextrin molecules in a skewered manner, and capping each end of the pseudopolyrotaxane with a capping group.
In the production method described in Patent Literature 1, the obtained polyrotaxane is purified by washing the polyrotaxane with a dimethylformamide/methanol mixed solvent, dissolving the polyrotaxane in dimethyl sulfoxide, dropping the resulting solution in water to precipitate the polyrotaxane, and separating the solid and liquid phases by centrifugation, whereby free cyclodextrin molecules which deteriorate the characteristics of a crosslinked polyrotaxane are removed.